1. Field of the Invention
This invention relates generally to a system for recording and reproducing a composite television signal containing a video signal component and a sound signal component, and more particularly to a system for achieving long-time recording of the television signal for the purpose of long-playback from a given amount of recording medium.
2. Description of the Prior Art
Various systems have been proposed for recording a television signal on a recording medium such as a magnetic tape, rotatable magnetic sheet, rotatable optical disc and the like, and reproducing the recorded television signal in the composite form or in the form of separated video signal, sound signal and other signals. To cite an example of the system utilizing the rotatable optical disc, generally the rotating speed of the disc is such that each frame interval of the television signal is optically recorded on the rotating disc at each turn of the latter and, as a result, a plurality of one-frame circular tracks are formed on the disc with a predetermined space between adjacent tracks. Actually, each of these tracks spirals into the next one to form a single recorded spiral track inclusively. In reproducing, the recorded disc is turned at the same speed as in recording and each recorded frame interval of the television signal is reproduced at each turn of the disc sequentially from the recorded spiral track.
In such a previously proposed system, a certain annular area on the surface of the rotatable disc is used at every turn of the disc in forming a single circular track, on which one frame interval of the television signal is recorded, together with the space provided for separating the track from adjacent circular tracks. Accordingly, the total recording time is restricted by the dimensions of that portion of the surface of the disc available for forming the recording track. Generally, it is difficult to allow the diameter of the rotatable disc to be immoderately large and it is restricted to be, for example, 10 to 15 inches. As a result of this, the total recording time is limited to a short duration of, for example, 30 minutes for a disc of 12 inch diameter.
To increase the recording time, some attempts have been proposed in prior art recording and reproducing systems. According to one of those attempts, the rotational speed of the disc is reduced enough to record several frame intervals of the television signal on a single circular track. According to another attempt, the space between each pair of adjacent circular tracks, that is, the pitch of the spiral track is reduced so as to increase the track density on the surface of the disc. These attempts, however, require the recording mechanism to have extremely accurate operation with very complicated and sensitive controls. Such mechanisms can only be produced with many difficulties. In addition, they are likely to cause deteriorations in the quality of reproduced signals. Therefore, satisfactory lengthening of the recording time has not been accomplished by the previously proposed attempts.